1. Field of the Invention
This invention pertains to the field of automatic transmissions for automotive vehicles. More particularly, the invention pertains to a technique for assembling a friction element with a predetermined preload in a spring subassembly.
2. Description of the Prior Art
It is conventional practice in an automatic transmission to control the operation of a gearset using hydraulically-actuated friction elements, i.e., friction clutches and brakes. A friction element includes an hydraulic cylinder fixed against displacement on a hub, a piston axially displaceable within the cylinder, a balance piston located within the cylinder, and a compression spring pack subassembly contacting the balance piston and piston. A snap ring is used to join the balance piston to the hub.
A spring force applied to the piston assembly forces the piston away from the balance piston and into contact with the hydraulic cylinder. When the friction element is disengaged, pressurized hydraulic fluid is vented from the cylinder and the spring pack forces the piston away from sets of clutch discs whose mutual, frictional engagement produces a driving connection between the elements of the clutch or brake. To engage the friction element, pressurized hydraulic fluid is supplied to the hydraulic cylinder between the piston and cylinder, thereby producing a pressure force on the end of the piston, which acts to compress the spring and to force the sets of clutch discs into mutual frictional engagement.
It is important in the operation of the friction element that the hydraulic piston quickly take up clearances in the clutch or brake and bring the friction discs into mutual contact. To accomplish this, high pressure hydraulic fluid is applied to the clutch cylinder for a brief period, after which hydraulic pressure magnitude is decreased abruptly and then increases at a linear rate over time. These details are directed to accomplish the gear ratio change quickly and smoothly without harshness or excessive wear, and preferably without the vehicle operator being able to perceive the gear ratio change.
When the compression preload in the spring assembly is low, clearances in the friction element are taken up quickly and the gear ratio change is completed sooner than if the spring pack preload were higher than needed to return the hydraulic piston into contact with the hydraulic cylinder.
Automatic transmission shift quality is dependent upon the consistency of spring pack preload. Variation in the spring pack free length and clutch housing dimensional consistency results in a large variation in the magnitude of the spring pack preload.